1. Field of the Invention
This invention relates to an etching method and a manufacturing method of a structure, especially suitable for application to fabrication of fine micro structures including those in the technical field of micro electromechanical systems (MEMS), such as micro mirrors.
2. Description of the Related Arts
In the technical field of MEMS, it has been known that any trench made to pass through silicon (Si) substrate by dry etching is inevitably enlarged in width at its terminal end (for example, Transducers ""99: J. JIAO et al. pp.546-549). This is caused by an increase of the etching speed at the terminal end of the trench.
When a trench increases its width at the terminal end, a trench configuration, as designed, cannot be obtained, and this disturbs fabrication of micro structures in the field of MEMS.
Moreover, since any etching normally has a difference of xc2x15% in rate within a wafer surface, almost no over-etching is conducted throughout the entire surface of the wafer. It is not possible to solve the above-indicated problem by means of so-called just etching.
It is therefore an object of the invention to provide an etching method and a manufacturing method of a structure using such etching method which are usable for making a trench extending through a substrate by dry etching, or more generally, for making a through hole in a member to be etched by dry etching, and can prevent the through hole from enlarged in dimension at its terminal end.
The Inventor made careful researches toward solution of the above-indicated problem involved in conventional techniques. These researches are outlined below.
The Inventor experimentally made a pass-through trench by dry etching of a Si substrate. More specifically, a 10 xcexcm wide passing-through trench in a 100 xcexcm thick Si substrate. FIG. 1 is a photomicrograph of the trench formed in the Si substrate taken from its etching-start side, i.e. its side having a resist mask. As shown in FIG. 1, the width of the passing-through trench observed from the etching-start side was approximately 10 xcexcm. On the other hand, FIG. 2 shows a photomicrograph of the passing-through trench taken from the etching-terminal side and having the same magnification as that of FIG. 1. As shown in FIG. 2, the maximum width of the passing-through trench was approximately 30 xcexcm at the etching-terminal end, and it was approximate three times the width there of in the etching-start end.
This result of experiments coincided with the result of experiments made by J. JIAO et al. in the point that the terminal end of a pass-through trench was enlarged in width. However, it was probably first found that the width of a pass-through trench increases at its terminal end as large as three times the width at its etching-start side.
The Inventor made a review about the reason of those experimental results. That is, as understood by carefully observing the photomicrograph shown in FIG. 2, the terminal end of the passing-through trench have two portions which are narrower than their neighbor portions, and they are approximately equal to the width of the passing-through trench at its etching-start side. It should be remarked here that, at opposite sides of such portion of the passing-through trench, an Al film stacked on the bottom surface of the Si substrate remains non-etched (which appears white in FIG. 2). This fact can be interpreted to indicate that existence of an Al film on the back surface of a Si substrate near the portion where a pass-through trench passes through is effective for preventing enlargement in width of the passing-through trench at its terminal end.
It is currently unknown what mechanism produces such results. However, the following mechanism can be presumed. That is, as a reason of enlargement in width at the terminal end of the trench made to pass through a Si substrate by dry etching, side walls of the passing-through trench might be electrically charged while the etching progresses, which might bend orbits of ions coming later; ions hitting against the side walls of the trench might increase their speed components normal to the side walls at the terminal end of the trench. In contrast, in the case where an Al film is formed on the bottom surface of the Si substrate near the portion of the terminal end of the passing-through trench, when etching progresses to the terminal end of the trench and the Al film is exposed inside the trench, a charge might quickly move from side walls of the trench onto the Al film and thereby completely cancel, or largely reduce, the charge of the side walls of the passing-through trench, which might considerably alleviate deviation of orbits of subsequent ions, and might prevent that the etching rate increases at the terminal end of the passing-through trench.
Taking those mechanisms into consideration, the Inventor made a further review, and reached to the conclusion that, by previously making a conductor with a sufficiently high electric conductivity on one side corresponding to the terminal end of the trench, more typically, by locating a conductor in contact with the surface corresponding to the terminal end of the trench by any appropriate means, the same effect could be obtained. Through a still further review, the Inventor has come to the conclusion that this technique is effective not only for cases where the entity to be etched is the Si substrate, but also for all cases involving the problem that the terminal end of a through hole is enlarged in size when it is made by etching an object to be etched by dry etching.
The present invention has been made through those studies and reviews by the Inventor.
According to the first aspect of the invention, there is provided an etching method configured to make a through hole by etching an object to be etched from one of major surfaces thereof by dry etching, comprising:
the dry etching being conducted under the condition where a conductor with a higher electric conductivity than that of the entity is in contact with the entity at least in or near a location for making the through hole.
According to the second aspect of the invention, there is provided a manufacturing method of a structure including a step of making a through hole by etching an object to be etched from one of major surfaces thereof by dry etching, comprising:
the dry etching being conducted under the condition where a conductor with a higher electric conductivity than that of the entity is in contact with the entity at least in or near a location for making the through hole.
In the present invention, the entity to be etched may be essentially any that involves the problem of undesirable enlargement in size of a terminal end of a through hole made therethrough by dry etching. Specifically, it may be any semiconductor, for example, among Si, and element semiconductors or compound semiconductors like Ge, SiGe and GaAs, for example. Further, the entity to be etched may be a single-layered film, such as Si substrate, which is made of a single kind of substance, or may be a multi-layered structure stacking multiple films made of two or more kinds of substances, such as a multi-layered film stacking two Si films via a SiO2 film, for example. In such a multi-layered structure, it is often desired to pierce only a certain layer through by etching. Also in that case, since the through hole may enlarge at boundaries if respective layers are different in etching rate, the effect of preventing undesirable enlargement of the through hole at its terminal end can be obtained by merely providing a conductor at a location where the etching of the layer should be stopped.
Preferably used as the conductor is one having a sufficiently high electric conductivity as compared with the entity to be etched. More specifically, any of Al, Au, Pt, Ti, Cr, W and Mo, for example, may be used as the conductor. Alternatively, a compound of a conductive metal like metal silicide, electrically conductive organic matter like conductive polymer, or other like material, can be used as the conductor. This conductor may be a conductive film formed on the entirety of the other surface of the entity to be etched, or may be a conductive film in form of a fine-line pattern, for example, formed near the site for making the through hole on the other surface of the entity to be etched. The latter form of conductive film is more advantageous because its coverage may be so small that its presence on the final structure does not invite problems, and it need not be removed by a later step after etching. In the case where the entity to be etched is a semiconductor such as Si, it is also possible to introduce an n-type impurity or a p-type impurity onto the other surface of the entity to be etched by ion implantation and use the impurity-introduced layer as the conductor.
The conductor is not limited to those made by film-making techniques, but may be a molten conductor such as molten metal. Metals suitable for this purpose are, for example, low-melting-point metals like Ga, Hg, solder and In, for example. If such a molten conductor is used, etching can be conducted by the following steps, for example. First, a low-melting-point conductor is set on a wafer stage in a dry etching apparatus. Then, after maintaining the wafer stage at a temperature above the melting point of the conductor and melting the conductor, a wafer (such as Si substrate) as the entity to be etched is put thereon. Subsequently, after fixing the wafer on the wafer stage by decreasing the temperature of the wafer stage below the melting point of the conductor, the through hole is made by dry etching of the wafer. In certain cases, the through hole may be made by melting the conductor by maintaining the wafer stage at a temperature above the melting point of the conductor, then putting thereon the wafer as the entity to be etched, and conducting dry etching of the wafer.
The dry etching may be essentially any that involves the problem of undesirable enlargement of the terminal end of a through hole made by etching the entity to be etched. For example, it may be an ion-used dry etching such as reactive ion etching (RIE), for example. Especially when the entity to be etched is Si, SF6 gas and C4F8 gas are used as the etching gas.
Regarding the aspect ratio of the through hole (=(vertical size)/(horizontal size)) to be made through the entity to be etched, there is no essential limitation. Typically, however, it is 3 or larger, with which it is considered difficult for conventional dry etching to make a through hole uniform in size throughout its entire length including its terminal end. It is preferably not smaller than 5, more preferably not smaller than 8 and more preferably not smaller than 10. The upper limit of the aspect ratio will be determined mainly depending upon the performance of the dry etching apparatus used, and it is not definite. However, a criteria is about 20 to 50.
The aspect ratio of a through hole in a multi-layered film as the entity to be etched is given as the ratio of the vertical size of the through hole with respect to its horizontal size when viewed from the entirety of the multi-layered film.
As summarized above, according to the invention, since a through hole is made by conducting dry etching while a conductor with a higher electric conductivity than that of an object to be etched is held in contact with the other surface of the entity to be etched in or near the location for making the through hole, when the etching progresses to the terminal end of the through hole and the conductor is exposed inside the through hole, charges electrically charged by ion injection quickly move from side walls of the through hole onto the conductor, thereby completely or significantly remove the electric charge of the side walls inside the through hole, and thereby significantly decrease deviation of orbits of subsequent ions. Therefore, acceleration of the etching rate at the terminal end of the through hole is effectively prevented.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.